The field of the present invention is screening systems including the use of both low and high frequency vibration.
Traditional vibratory screening structures typically include a base, a housing resiliently mounted on the base with a screen or screens extending across the housing, a dome beneath the screen to direct the screened material to the periphery of the housing and outlet ports above and below the screen for oversized and screened particles, respectively. A low frequency vibratory drive in the speed range of 8 Hz to 30 Hz is mounted to the housing and drives eccentric weights. Specific vibratory motions are established in the housing by the low frequency vibratory drive, generating screen accelerations up to the 7 g range.
The foregoing devices have been used for screening all sizes of materials and powders. For fine materials and powers, stainless steel woven mesh screens having interstices in the 30 to 150 micron range are used for commercial processing. These delicate, woven meshes are thin and comparatively limp. The mesh is usually stretched tightly and attached to a screen frame. The vibration of such devices typically enhances gravity separation of particles presented to the screen. Where fine particles are to be screened, the vibration also can have a deleterious effect in that the fine particles become suspended above a boundary layer over the vibrating screen.
In an effort to overcome the deficiency of low frequency vibration, high frequency vibration has been employed. Ultrasonic vibrators such as magnetostrictive ultrasonic transducers have been mounted to separator housings, to peripheral screen frames and directly to screens at the centers thereof. Multiple such ultrasonic vibrators have been employed for better energy distribution. FIGS. 1 through 5 represent devices preceding the present invention.
FIG. 1 illustrates a prior vibratory screen separator. The separator includes a base 10 resiliently mounting a separator housing 12 by means of springs 14. The separator housing 12 is illustrated here to be cylindrical, open at the top to receive material input and having discharge ports 16 and 18 for the screened material and the oversize material, respectively. A low frequency vibratory drive 20 is rigidly fixed to the separator housing 12. The drive 20 includes upper and lower eccentric weights 22 and 24 to generate a vibratory motion when rotatably driven.
A screen 26 extends across the separator housing 12 such that material input above the screen 26 must either pass through the screen or through the oversize discharge port 18. The screen 26 includes a screening element 28 stretched in tension uniformly by a screen frame 30. The screening element 28 is a composite of a fine mesh screen of a desired size with a stiffer porous sheet, most conveniently a perforated plate. Diffusion bonding is employed across the full area of the screening element 28. Such bonded screening elements are commercially available.
To bond fine mesh screen cloth to a perforated plate with diffusion bonding, it is preferable to include a coarse mesh screen cloth therebetween. A fine mesh screen 32, a coarse mesh screen 34 and a perforated plate 36 are shown. These are supplied as a diffusion bonded laminate which is tensioned and bonded to a screen frame 30 to form the screen structure 26. The fine mesh screen cloth may be dictated by the requirements of the materials being screened. 200 mesh and 325 mesh screen cloth is common. The backing perforated plate is preferably 80% open and is from 1/16" to 3/16" thick. Design choice may dictate thinner or thicker plates depending on separator size, weight of material, degree of low frequency vibrations and the like.
As illustrated in FIGS. 2 through 5, two types of screen mountings have been employed. In FIGS. 2 and 3, the separator housing 12 directly supports the screen 26. The high frequency generator 38 is shown to be mounted rigidly to the separator housing 12 by a fixed bracket 40. The action of the high frequency generator or generators 38 through the fixed bracket or brackets 40 on the separator housing 12 is transferred from that peripheral housing to the screen 26. The separator housing 12 in its entirety is also subject to be vibrated in this arrangement.
An alternative arrangement is illustrated in FIGS. 4 and 5. In these Figures, resilient elements or gaskets 42 and 44 are positioned on top and bottom of the screen frame 30 to isolate the screen frame from the surrounding separator housing 12. The resilient elements would act to isolate the separator housing 12 but would also act to damp some of the power generated by the generator(s) 38. Again, the vibration is introduced by means of a peripheral frame to the screen 26.
Low frequency vibration is employed at levels allowing the separator housing 12 and screen 26 to vibrate as a rigid body, even in the embodiment where the resilient elements 42 and 44 are employed. Smaller power and lighter weights may be employed for the low frequency vibratory drive as compared with conventional low frequency separators since this drive is now relegated to transportation of material across the screen. Low frequency vibrations effective for conveying material are typically considered most effective in the 3 Hz to 30 Hz range. Energy typically effective for conveyance of fine material on a screen may be measured in screen acceleration in the 1/2 g to 4 g range at 20 Hz.